The present invention generally relates to a method for forming quaternary composite films of Coxe2x80x94Wxe2x80x94Pxe2x80x94Au and devices formed containing such films and more particularly, relates to a method for forming quaternary films of Coxe2x80x94Wxe2x80x94Pxe2x80x94Au by first forming a Coxe2x80x94Wxe2x80x94P alloy film and then electrolessly depositing a Au film on top of the alloy film such that Au diffuses into the alloy film forming the quaternary composite film and devices formed containing such film.
The technology of making interconnections for providing vias, lines and other recesses in semiconductor chip structures, flat panel displays, and package applications has been developed for many years. For instance, in developing interconnection technology for very-large-scale-integrated (VLSI) structures, aluminum has been utilized as the primary metal source for contacts and interconnects in semiconductor regions or devices located on a single substrate. Aluminum has been the material of choice because of its low cost, good ohmic contact and high conductivity. However, pure aluminum thin-film conductors have undesirable properties such as a low melting point which limits its use to low temperature processing, possible Si diffusion into Al during annealing which leads to contact and junction failure, and poor electromigration resistance. The electro migration phenomenon occurs when the superposition of an electronic field onto random thermal diffusion in a metallic solid causes a net drift of ions. Consequently, a number of aluminum alloys have been developed which provided advantages over pure aluminum. For instance, U.S. Pat. No. 4,566,177 discloses a conductive layer of an alloy of aluminum containing up to 3% by weight of silicon, copper, nickel, chromium and manganese to improve electromigration resistance. U.S. Pat. No. 3,631,304 discloses aluminum alloys with aluminum oxide which were also used to improve electromigration resistance.
More recently VLSI and ULSI technology has placed more stringent demands on the wiring requirements due to the extremely high circuit densities and faster operating speeds required of such devices. This leads to higher current densities in increasingly smaller conductor lines. As a result, higher conductance wiring is desired which requires either larger cross-section wires for aluminum alloy conductors or a different wiring material that has a higher conductance. The obvious choice in the industry is to develop the latter using copper based on its desirable high conductivity.
In the formation of VLSI and ULSI interconnection structures such as vias and lines, copper is deposited into a line, via or other recesses to interconnect semiconductor regions or devices located on the same substrate. Copper is known to have problems at semiconductor device junctions due to its fast reaction rate with Si. Any diffusion of copper ions into the silicon substrate can cause device failure. In addition, pure copper does not adhere well to oxygen containing dielectrics such as silicon dioxide and polyimide.
It is therefore an object of the present invention to provide a diffusion barrier layer between a copper interconnect and other semiconductor materials that does not have the drawbacks or shortcomings of the conventional diffusion barriers.
It is another object of the present invention to provide a diffusion barrier layer between a copper interconnect and a silicon substrate in a semiconductor structure.
It is a further object of the present invention to provide a diffusion barrier between a copper interconnect and a dielectric material layer in which the interconnect is formed.
It is another further object of the present invention to provide a diffusion barrier layer for a copper interconnect in a semiconductor structure wherein the barrier layer is a quaternary composite film of Coxe2x80x94Wxe2x80x94Pxe2x80x94Au.
It is still another object of the present invention to provide a diffusion barrier layer for a copper interconnect in a semiconductor structure wherein the barrier layer consists of a Au film coated on a Coxe2x80x94Wxe2x80x94P alloy film.
It is yet another object of the present invention to provide a diffusion barrier layer for a copper interconnect in a semiconductor structure which is formed by two sequential electroless plating processes.
It is still another further object of the present invention to provide a method for forming a quaternary Coxe2x80x94Wxe2x80x94Pxe2x80x94Au composite film by first electroless plating a Coxe2x80x94Wxe2x80x94P film on the surface of a copper conductive region in a plating bath containing cobalt ions, tungstate ions, citrate ions and a reducing agent, and then immersing the substrate in a Au electroless plating solution for depositing a Au layer on top.
It is yet another further object of the present invention to provide an electronic structure which includes a Coxe2x80x94Wxe2x80x94Pxe2x80x94Au composite film coating in a via opening that is filled with copper for forming a copper interconnect.
In accordance with the present invention, a method for forming a diffusion barrier layer for a copper interconnect in a semiconductor structure and the structure formed are disclosed.
In a preferred embodiment, a method for forming a Coxe2x80x94Wxe2x80x94Pxe2x80x94Au composite film on a substrate can be carried out by the operating steps of cleaning a substrate which has a copper conductive region in 0.2 M H2SO4 for 10xcx9c20 sec., then rinse in H2O for 60 sec., the substrate is then immersed in a solution that contains palladium ions for a length of time sufficient for palladium metal to deposit by a redox reaction on surfaces of the copper conductive regions, rinsing the substrate in H2O, immersing the substrate in a solution containing at least 15 gr/l sodium citrate or EDTA (Ethylene diaminetetra acetic acid) complexing agents for removing excess palladium ions from the surfaces of the copper conductive regions, rinsing the substrate with distilled water, electroless plating a Coxe2x80x94Wxe2x80x94P film on the surfaces of copper conductive regions in a plating solution containing cobalt ions, tungstate ions, citrate ions and a reducing agent, rinsing the substrate with distilled water, and immersing the substrate in a Au electroless plating solution for depositing a Au layer on top of the Coxe2x80x94Wxe2x80x94P film alloy.
In the method for forming a Coxe2x80x94Wxe2x80x94Pxe2x80x94Au film on a substrate, the first and second immersing steps are carried out at ambient temperature. The method may further include the step of mixing a first plating solution of cobalt ions/tungstate ions at a ratio of between about 1 and about 10, stabilizing the plating solution with citrate ions at a citrate ions/cobalt ions ratio of not less than 3, adjusting the pH of the plating solution in the range between 7 and 9 by using a NaOH and boric acid buffer and adding a hypophosphite reducing agent. The method may further include the step of maintaining the first plating solution at a temperature between about 65xc2x0 C. and about 85xc2x0 C. The method may further include the step of mixing the first plating solution of cobalt ions/tungstate ions preferably at a ratio of between about 2 and about 4. The method may further include the step of stabilizing the first plating solution with citrate ions at a citrate ions/cobalt ions ratio preferably not less than 5. The method may further include the step of maintaining the first plating solution at a temperature preferably between about 70xc2x0 C. and about 80xc2x0 C.
In the method for forming a Coxe2x80x94Wxe2x80x94Pxe2x80x94Au film on a substrate, the process for forming by adding boric acid at a concentration of at least three times that of the cobalt ions, and adding a surface active agent. The method for forming the first plating solution may further include the step of adding a reducing agent containing hypophosphite at a concentration of at least 1.2 times that of the cobalt ions. The method may further include the step of immersing the substrate in a solution containing preferably at least 30 gr/l of sodium citrate or ethylenediamine tetra acetic acid, sodium salt (EDTA) for removing excess palladium ions.
In the step of electroless plating a Coxe2x80x94Wxe2x80x94P film on the substrate, the Coxe2x80x94Wxe2x80x94P film produced has a thickness between about 100 xc3x85 and about 1500 xc3x85. The electroless plating process can be carried out at a depostion rate of not less than 0.5 xcexcm/hr, and preferably at not less than 0.8 xcexcm/hr.
In the method for forming a Coxe2x80x94Wxe2x80x94Pxe2x80x94Au film on a substrate, wherein the electroless Au plating step is carried out in a plating solution which has a pH between about 7.0 and about 8.5. The Au plating step may be carried out in a cyanide-free, low pH plating solution. The electroless Au plating step may be carried out in a plating solution which contains a Au salt, a sulfite-thiosulfate electrolyte and a reducing agent. The method may further include the step of depositing a Au layer on top of the Coxe2x80x94Wxe2x80x94P film to a thickness of between about 200 xc3x85 and about 2,000 xc3x85. The Coxe2x80x94Wxe2x80x94P film formed may include between about 85% and about 95% cobalt, between about 1% and about 5% tungsten and between about 5% and about 9% phosphorous. The step of plating the Coxe2x80x94Wxe2x80x94P film produces a film preferably having a thickness of between about 200 xc3x85 and about 600 xc3x85. The method may further include the step of annealing the Coxe2x80x94Wxe2x80x94P film at a temperature of between about 200xc2x0 C. and about 350xc2x0 C. in an inert atmosphere such as a forming gas. Annealing step may also be carried out under similar conditions after deposition of the Au film.
The present invention is further directed to an alloy film which includes a Coxe2x80x94Wxe2x80x94P film layer, and a Au layer on top of the Coxe2x80x94Wxe2x80x94P film layer with Au atoms diffused into the Coxe2x80x94Wxe2x80x94P film layer forming a Coxe2x80x94Wxe2x80x94Pxe2x80x94Au composite film. The Coxe2x80x94Wxe2x80x94P film layer may consist of between about 85% and about 95% Co, between about 1% and about 5% W, and between about 5% and about 9% P. The Au layer may have a thickness between about 200 xc3x85 and about 2,000 xc3x85.
The present invention is still further directed to a Coxe2x80x94Wxe2x80x94Pxe2x80x94Au film which includes between about 90% and about 95% Co, between about 1% and about 3% W, between about 3% and about 5% P, and between about 1% and about 2% Au. Sample compositions are 90% Co, 3% W, 5% P and 2% Au; or 95% Co, 1% W, 3% P and 1% Au.
The present invention is further directed to an electronic structure which includes a semi-conducting substrate, a dielectric layer deposited on top of the substrate, a via opening in the dielectric material layer exposing the substrate, a conformal coating layer of Coxe2x80x94Wxe2x80x94P alloy in the via opening, and a copper interconnect in the via opening, whereby the conformal coating layer of Coxe2x80x94Wxe2x80x94P alloy on copper is a diffusion barrier for preventing copper from diffusing into the semi-conducting substrate and the dielectric material layer.
In the electronic structure, the semi-conducting substrate may be a silicon substrate, the dielectric layer may be a SiO2 layer. The structure may further include a Au layer on top of the Coxe2x80x94Wxe2x80x94P conformal coating layer.
The present invention is still further directed to an electronic structure which includes an insulating material layer that has a top surface, at least one copper bond pad formed in a top surface of the insulating material layer, and a layer of Coxe2x80x94Wxe2x80x94Pxe2x80x94Au composite film overlying the at least one copper bond pad.
In the electronic structure, the Coxe2x80x94Wxe2x80x94Pxe2x80x94Au film layer may be formed by a Au layer deposited on top of a Coxe2x80x94Wxe2x80x94P alloy layer. The structure may further include at least one solder ball on top of the at least one copper bond pad with the Coxe2x80x94Wxe2x80x94Pxe2x80x94Au composite layer thereinbetween. The electronic structure may further include a ball-limiting-metallurgy (BLM) layer on top of the Coxe2x80x94Wxe2x80x94Pxe2x80x94Au layer, and at least one solder ball planted on top of the BLM layer, the Coxe2x80x94Wxe2x80x94Pxe2x80x94Au layer and the at least one copper bond pad.